As one of lithography processes in a semiconductor process, there is a pattern transfer technique using an EUV (Extreme Ultra Violet) light beam. This technique has attracted attention as one of methods capable of forming finest patterns among recent lithography techniques.
In the lithography using an EUV light beam, since the EUV light beam may not transmit through glass, used is a reflection-type photomask (EUV mask) where the EUV light beam is reflected by an multi-layered film obtained by alternately forming Mo (molybdenum)/Si (silicon). In terms of the configuration of the apparatus, the rear surface side of the EUV mask is suctioned by using an electrostatic chuck method or the like, so that the EUV mask is retained inside the EUV exposure apparatus.
However, in the case where the rear surface side is suctioned, there are problems in that insertion of particles (fine dust) attached to the suction sites leads to a deterioration in flatness or in that a difference in suction force causes occurrence of horizontal distortion in the mask. Even in the case where the distortion occurs, if the amount of distortion can be accurately measured, the distortion amount is corrected and exposure can be performed. Therefore, it is preferable that the distortion amount be accurately measured.